Revisiting the Early Collisional Evolution of the Moon

In yet another example of the continued collaboration among the teams of the NASA Lunar Science Institute and the Observatoire de la Côte d’Azur in Nice, France, Alessandro Morbidelli, Simone Marchi, William Bottke, and David Kring have published a paper in the Journal Earth and Planetary Science Letters.

The researchers revisited the early evolution of the Moon’s bombardment. Using the best available dynamical models of terrestrial planet formation and giant planet evolution, they predicted the lunar impact flux during its earliest stages, ranging from tens of millions of years after the Moon’s formation to a billion years later (i.e., from 3.5-4.5 billion years ago). This model covers the so-called period of late lunar bombardment when many of the largest and oldest lunar impact craters were formed.

By calibrating their results against existing dynamical, geochemical and crater density constraints, the team obtained a self-consistent picture of the early collisional evolution of the Moon. Specifically, the team combined their dynamical modeling work with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile, or iron-loving, elements like gold, platinum, osmium and iridium in the lunar crust and mantle.

The team deduced that the evolution of the impact flux did not decline as rapidly as previously thought over the first billion years of lunar history, and that there was no single, “narrow” impact spike some 3.9 Gy ago, as is often shown in graphical sketches of the lunar late heavy bombardment. Instead, the bombardment timeline that emerged from their study showed something of a “sawtooth” profile, with an uptick in the impact flux about 4.1 Gy ago.

The sawtooth-like bombardment timeline has important implications for Earth’s habitability. In previous views, the Earth was either an increasingly hostile abode for life going back in time toward the Moon forming event, or it was relatively tranquil for several hundreds of million of years just prior to the hypothesized impact spike that took place 3.9 billion years ago. In the new sawtooth view, big impactors hit over an extended period, with more lulls and therefore more opportunities for the biosphere to recover. Perhaps in this scenario, life formed very early and has survived in one form or another through the late heavy bombardment.

The contributions of S. Marchi, W. F. Bottke, and D. A. Kring were supported by NASA’s Lunar Science Institute (Center for Lunar Origin and Evolution at the Southwest Research Institute in Boulder, CO; Center for Lunar Science and Exploration at the Lunar and Planetary Institute and Johnson Space Center in Houston, TX).